The invention relates to a process for winding a continuously supplied band onto a bobbin, with the bobbin being rotated and the band being reciprocated along the entire length of the bobbin at a winding angle by means of a cross-winding device, wherein each time the bobbin diameter has increased by a particular value, the winding ratio, i.e. the ratio between the number of bobbin rotations and the reciprocating motion (to-and-fro stroke) of the cross-winding device, is changed in steps.
Among experts, such a process for winding a continuously supplied band is referred to as “stepped precision winding” and is known, for instance, from DE 41 12 768 A, DE 42 23 271 C1 and EP 0 561 188, the latter providing a detailed general account of various types of bobbin shapes.
The band is wound onto cylindrical or conical bobbin cores in winding machines, whereby the speed of supplying the band to the bobbin core is relatively constant, since it has been predetermined by band-manufacturing machines provided upstream of the winding machine.
The appearance, strength and quality of the bobbins is strongly affected by the following parameters:                1) The winding angle α, which is the angle between a normal line to the axis of rotation of the bobbin and the longitudinal direction of the band supplied to the bobbin.        2) The winding ratio V, which is the number of bobbin rotations per to-and-fro stroke of the cross-winding device.        
The winding angle α arises from the selected winding ratio V.
Stepped precision winding is a mixture of two basic winding methods of how to wind the supplied band onto a bobbin core, namely between “random winding” and “precision winding”.
The characteristic feature of random winding is a constant winding angle α contrasted by a variable ratio between the number of bobbin rotations and the traverse speed (=variable winding ratio V). In the winding ratio/bobbin diameter chart of FIG. 2, three graphs are plotted for random windings with winding angles α=4°, 5°, 6°. One advantage of random winding is the simple design of the winding machine necessary for its generation, which is illustrated in side view and top view in FIG. 3. In the most simple case, it may comprise a motor 10 actuating a driving roller 11 which, in turn, engages the periphery of the bobbin 12, driving the same at a constant peripheral speed so that the band 19 is wound up at a constant linear speed. The bobbin spindle 18 of the bobbin 12 may be configured so as to run freely. Via a transmission gear consisting of pulleys 15, 16 and a belt 17 running over the two pulleys, the motor 10 actuates a cross-winding device 13 in such a way that the traversing band guide 14, through which the band 19 passes, will reciprocate at a constant stroke speed (traverse stroke). Hence, there is a fixed transmission ratio between the peripheral speed of the bobbin 12 and the traverse stroke of the traversing band guide 14, resulting in a constant winding angle of the band 19 on the bobbin 12. This means that the winding angle at the beginning of the process of winding onto an empty bobbin core is the same as at the end of the winding process when the bobbin has reached its maximum diameter. Disadvantageously, the number of windings per winding layer thereby decreases steadily as the bobbin diameter increases so that a bobbin is created whose band material has a different packing density at every bobbin diameter. Another adverse effect occurring during winding, referred to as “pattern development”, arises at certain ratios between bobbin diameters and traverse speeds, whereby, at those ratios, several layers of bandlets are superimposed almost exactly, thereby rendering the bobbin unstable. Therefore it is necessary to take measures to create “pattern interference”, f.i. wobbling.
Precision winding, on the other hand, is characterized by a constant winding ratio along the entire increasing bobbin diameter, which in turn means that the winding angle will decrease as the bobbin diameter increases. In the chart of FIG. 2, a precision winding with a winding ratio V=35 is plotted as a straight line. The advantage of precision winding lies in achieving a bobbin whose band material has a constant packing density on the bobbin independently of the bobbin diameter. The disadvantage of precision winding is that—starting from an initial winding angle at the beginning of winding the band material onto an empty bobbin core—the winding angle gets smaller and smaller as the bobbin diameter increases and finally is so small (theoretically approaching zero) that the bobbin will become unstable. The design of a winding machine for generating a precision winding is illustrated in side view and top view in FIG. 4. The winding machine comprises a motor 20 rotating a bobbin spindle 21. A bobbin core 26 is fitted on the bobbin spindle 21 in torque-proof manner, on which core a band 27 is wound to form a bobbin 22. A cross-winding device 23 is connected with the bobbin spindle 21 via a spur gear 25. The cross-winding device 23 is equipped with rotation/translation converting means (not illustrated) for reciprocating the traversing band guide 24 in traverse strokes. By means of the direct rotary drive of the bobbin spindle 21, the rotational speed of the motor 20 must be steadily reduced as the diameter of the bobbin 22 being formed increases, since the band to be wound up is supplied by a band-manufacturing device at a constant linear speed.
So as to alleviate the respective disadvantages of random winding and precision winding and combine their advantages, the “stepped precision winding” was recommended in the past. The winding method is based on the concept that the winding ratio between predefined limiting diameters of a bobbin is kept constant and is changed in steps to a different value as soon as a respective limiting diameter has been reached, with the values of the winding ratios being chosen such that a graph of the winding ratio will roughly follow, across the bobbin diameter, the graph of a random winding for a particular winding angle. The advantage of stepped precision winding is that, on the one hand, “pattern development” is avoided since the volatile change of the winding ratio represents a “pattern interference measure”.
On the other hand, the winding angle does not become substantially smaller than the initial winding angle even if the bobbin diameter increases.
While the stepped precision winding yields the expected good result for the manufacture of yarn and thread bobbins, surprisingly poor results are often achieved if band bobbins are produced by stepped precision winding. The inadequacies of those band bobbins range from an irregular and therefore unsightly optical appearance to bobbins with varying, f.i. corrugated, diameters throughout their lengths, from irregular spindle fronts to an unstable winding structure.
Since such bobbins are usually used in rapidly operating machines such as circular looms, each irregularity of the bobbin structure can have fatal results, which, as the smallest consequence, will result in the rupture of the band as it is drawn off from the bobbin and, in the worst case, will involve the destruction of a part of the machine. Such damages are caused by unbalanced masses at irregular bobbins, by vibrations in the bands that gradually build up as they are drawn off, etc. Furthermore, irregular bobbins will heat up rapidly if the bands are drawn off quickly, thus leading to fatigue and weakening of the band material, in particularly if the material is oriented plastic bands.
For that reason, a strong demand for an improved process of stepped precision winding exists in the industry.